Dr. H. Dor on Vision in the Arthropoda. 235 
eye, perfectly analogous to the simple eye of the Vertebrata. The 
lenticular cornea corresponds with the cornea and the crystalline 
apparatus ; the cone with the vitreous body; and the cupuliform 
envelope—which up to the time of Leydig was considered as con- 
junctive tissue, as neurilemma (J. Miiller), and which Leydig 
regards as an integral part of the bacillus—is for us a true retina, 
a dilatation of the optic nerve. We have not ascertained the 
presence of muscular fibres. The pigments replace the choroid, 
and the multiplicity of eyes the small muscles destined to move 
the eyes of the superior animals in various directions. If, as 
M. Claparéde points out with reason, there be some species in 
which Leuwenhoek’s images are not observed under the micro- 
scope, it must be owing to the convexity of their facets being 
very slight. The image is formed nevertheless, but much fur- 
ther backwards than it is usually looked for. We find such eyes 
in the Tabani; and in these the length of the crystalline bodies 
corresponds with the slight cur vature of the cornea, for they 
are about seven times as long as the thickness of the cornea, 
The mechanism of vision is therefore the same as in Man. One 
fact alone is not yet clear—namely, how distinct images can be 
formed upon a conical retina. But is the retina of Man always 
perfectly spherical? In any case, we no longer admit, with 
J. Miiller, that the only point sensible to light is the entrance of 
the optic nerve; for, to judge at least by analogy with the Ver- 
tebrata, this may very probably be the only blind point ; and we 
cannot understand how this distinguished physiologist could 
assume that, the pigment absorbing all the luminous rays which 
fall upon the sides of the cones, there could be no perception 
of light upon these points, as if the choroid in Man prevented 
the functions of the retina. 
“Lastly, it is not more difficult to explain simple vision with 
12,000 eyes, as in the Libellule, than with the two eyes of Man. 
Each eye in insects gives an image slightly different from that 
of the eye which is immediately in contact with it. But docs 
not Wheatstone’s admirable discovery, by throwing down the 
old doctrine of identical points, prove that the same thing takes 
place in our two eyes? It is owing to the presence of two dif- 
ferent images for the two eyes that we possess stereoscopic vision, 
that we appreciate distances, perspective, &c. This is the case 
also in insects, which will see the niore exactly in proportion as 
they have more facets. Thus the Ant, which moves slowly, does 
not require so exact an appreciation of distances as Butterflies, 
Dragon-flies, and other winged insects. For this reason probably 
we only find in the former 50 facets, whilst the Dragon-fly has 
12,000, and the Butterfly 17,000.” 
